1. Field of the Invention
This invention relates to an ink jet recording method for forming a full color image by use of two or more recording liquids (inks) with different colors including yellow, and more particularly to an ink jet recording method capable of forming a full color image having a broad color reproduction range and an abundant gradation characteristic and having good image stability, and also suitable for high speed recording.
2. Related Background Art
The ink jet recording method is a recording method in which recording is performed by generating small droplets of ink according to various ink discharging systems such as (1) the electrostatic attracting system by high voltage application, (2) the system in which mechanical vibration or displacement is applied to the ink by use of a piezoelectric element, (3) the system in which ink is momentarily foamed by heating and its pressure upon foaming is utilized, etc., and attaching such droplets onto a recording medium such as paper, etc.
The full-color recording is effected by subjecting the three colors of yellow (Y), magenta (M), and cyan (C), or four colors with black (K) added thereto to subtractive mixing of colors. Namely, full-color recording is effected by discharging the three colored inks or four colored inks with black (K) added thereto from nozzles used exclusively for the respective colors of the recording head while the respective amounts of the colored inks to be discharged are being controlled, and by allowing the respective inks to be mixed and absorbed by each picture element of the recording material.
For instance, in a conventional printer in which cylindrical piezoelectric elements are used in a printer head of an ink jet system, the gradations of density can be expressed by ink dot diameter modulation method in which the diameter of the dots to be printed on the recording sheet is modulated.
Meanwhile, since the ratio of change between a maximum dot and a minimum dot is not very large, there are cases where areas lack a sufficient density to output a natural color image. Accordingly, a low-density ink and a high-density ink both of the same color are prepared for each color, and the high- and low-density inks are selectively used by changing over the heads in accordance with the high- and low-density areas, thereby securing areas having necessary densities, as disclosed in, for example, U.S. Pat. Nos. 4,551,736, 4,533,920, 4,533,923 and 4,695,846. A method employing such high- and low-density inks for each color is hereinafter referred to as an ink density variation method.
However, when the formation of an image is carried out according to such an ink density variation method or an ink dot diameter modulation method, there occurs the phenomenon that particularly a skin color portion (a mixed color portion comprising yellow, magenta and cyan, i.e., highlight area) becomes yellow with the lapse of time.
This phenomenon, different from discoloration and fading phenomenon over a long time which is considered to occur owing to dyes contained in an ink, is a change in tone with the lapse of time in a short time scale. This phenomenon results in disintegration of the initial balance of an image and becomes a bar with respect to obtaining a full color image of high quality.
In addition, at a middle area, an image with clear tone and high contrast has not been obtained.